Its high switching speed, simple gate drive and wide safe operating area, have made vertical power MOSFETs a common switching device for modern power electronics. Because of its high current density, moderate switching speed, simple gate drive, and wide safe operating area, the IGBT is now also in widespread use for high power applications.
FIG. 1 illustrates a typical structure of vertical power MOSFET. The drain is at the bottom and the source is at the top. The current flows from bottom to top. Almost all of the area is used for current flow. The heat is dissipated through the entire bottom area, typically attached to a thick lead frame. In the blocking state, the voltage between the gate and the source is less than the threshold voltage and the N-drift layer holds the electric field. In the on-state, a positive voltage is applied between the gate and the source, allowing electron flow from source to the drain, as shown in FIG. 2.
FIG. 3 shows a top view of a typical vertical power MOSFET. The shaded region is the metal layer. The central big metal region is the active area in which the top metal layer is connected to the body and source of MOSFET. The outer metal loop is known as the gate bus line and is for controlling the gate voltage. Outside of the gate bus line is the junction termination area or ring area. This is for sustaining the high voltage in the blocking mode.
FIG. 4 shows the side view of the gate bus line and junction termination area. In the gate bus line area, the top metal is connected to the polysilicon gate. The polysilicon layer goes into the active area. In the junction termination area or ring area, there are P wells. As these P wells have a shape of a closed loop, they are often referred to as rings. These reduce the high electric field in the blocking mode, and make possible a high breakdown voltage.